Redimensione e recorte a imagem para o centro com PHP

<?php

    if(isset($_GET['src'])){

        $image = imagecreatefromjpeg($_GET['src']);
        $filename = 'anotherimagem.jpg';

        // Largura e altura desejada do thumbnail
        $thumb_width = 200;
        $thumb_height = 150;

        // Largura e altura da imagem em upload
        $width = imagesx($image);
        $height = imagesy($image);


        // Caluclos Matemáticos
        $original_aspect = $width / $height;
        $thumb_aspect = $thumb_width / $thumb_height;

        if ($original_aspect >= $thumb_aspect) {

           // If image is wider than thumbnail (in aspect ratio sense)
           $new_height = $thumb_height;
           $new_width = $width / ($height / $thumb_height);

        } else {

           // If the thumbnail is wider than the image
           $new_width = $thumb_width;
           $new_height = $height / ($width / $thumb_width);
        }

        $thumb = imagecreatetruecolor($thumb_width, $thumb_height);

        // Redimensionando e recortando
        imagecopyresampled($thumb,
                           $image,
                           0 - ($new_width - $thumb_width) / 2, // Center the image horizontally
                           0 - ($new_height - $thumb_height) / 2, // Center the image vertically
                           0, 0, $new_width, $new_height, $width, $height);

        // Definindo a qualidade da foto
        imagejpeg($thumb, $filename, 80);

    }

?>

<!DOCTYPE html>
<html>
<head>
    <title>Upload de imagens</title>
</head>
<body>
    <form method="GET" enctype="multipart/form-data">
        <input type="file" name="src" />
        <input type="submit" name="submit" value="Upload" />
    </form>
</body>
</html>